High purity titanium dioxide is an important commercial product, primarily because it is the predominant white pigment used for a wide range of products, including paints, white rubbers and plastics, paper, inks, etc. Titanium dioxide is obtained from various titanium-bearing ores, such as ilmenite, natural rutile, and leucoxene that occur as mineral sands and massive hard rock formations in many parts of the world. Rutile is the ore of choice for the production of high purity pigmentary grade titanium dioxide. The ores must be processed to remove impurities that darken the titanium dioxide crystals. These impurities include, inter alia, iron, chromium, vanadium, aluminum, and manganese and they must be removed in order for the titanium dioxide to be suitable for use as a high quality white pigment. One of the main commercial routes for the manufacture of high purity titanium dioxide is the co-called "chloride process". One of the first stages of the chloride process is carbothermal chlorination wherein the titanium-bearing ore is treated with carbon and chlorine at suitable process conditions to produce titanium tetrachloride and waste by-products comprised of metal chlorides. The titanium tetrachloride is purified and oxidized to produce high purity titanium dioxide.
The waste metal chlorides, which are acidic, must be disposed of in an environmentally friendly manner. It is well known that acidic waste metal chlorides may be dissolved in water and then treated, or rendered non-acidic, by reacting them with basic solutions, such as, for example, calcium hydroxide (milk of lime) or sodium hydroxide (caustic soda). The end products of treatment with such basic solutions are solid hydroxide precipitates of the metals of the metal chlorides and an aqueous solution of the salts of sodium chloride or calcium chloride. Non-limiting examples of metal hydroxide precipitates from such a process include ferrous hydroxide, aluminum hydroxide, magnesium hydroxide, chromium hydroxide, and manganese hydroxide. Few methods exist for acceptably disposing of the resulting metal hydroxide precipitates as well as the salt solutions. The most common method for disposing of metal hydroxides is to deposit them into landfills. Such deposits are typically not environmentally safe indefinitely. Long-term contact with naturally acidic rainfall could leach the metals from such deposits and thus have an adverse environmental impact on surrounding water and soil.
Another suggested method of disposing of metal hydroxide precipitates is to inject them as a water-borne slurry into deep porous underground strata. This method has not met with environmental approval because of concern that the slurry might contaminate subterraneous fresh-water aquifers.
Also, few methods have been devised for adequately disposing of the aqueous salt solution end products. If the metal chlorides have been neutralized with calcium hydroxide, the resulting salt in solution will be calcium chloride. Calcium chloride solution is sometimes dumped into brackish estuaries, or other marine bodies, where it is generally rendered harmless by dilution effects. However, in some instances salt concentrations could become excessively high, thereby creating environmental hazards. Furthermore, traditional desalination techniques that remove salts from water are costly and are extremely energy intensive.
If the metal chlorides have been neutralized with sodium hydroxide, the resulting salt in solution will be sodium chloride. There are currently operations in the United States that produce sodium chloride solutions, also referred to herein as "brine", as a result of the aforementioned treatment process. Some of these operations are inland where the brine cannot be dumped because it will harm freshwater environments. Consequently, the sodium chloride must be taken out of solution by boiling away the water and leaving behind the solid salt that may then be reused in any number of applications. There are clear disadvantages to such a salt disposal method. Desalination techniques, resulting in solid salts that are more easily disposable, are, as noted, extremely energy intensive and are not cost-effective.
Thus, a long-standing need has existed for a practical and environmentally acceptable method for disposing of both the metal hydroxide precipitates and the salt solution end products of the waste chloride treatment process.